The invention relates to singulating semiconductor wafers to form semiconductor chips. The process involves producing a semiconductor wafer having semiconductor chip positions arranged in rows and columns, rectilinear separating tracks being arranged between the semiconductor chip positions.
Usually, the semiconductor wafer is severed rectilinearly along the separating tracks by using a saw blade fitted with diamond chips. For this purpose, the saw blade has a metal film disk having a thickness of a few tens of micrometers that rotates at high speed, the metal film disk being supported by a saw blade body that is made to perform revolutions of more than 3000 revolutions per minute by an air-supported motor. On account of the disk-shaped metal film being covered with diamond chips, it is possible to introduce sawing grooves into the semiconductor crystal along the separating tracks with a width of less than 100 micrometers.
Such a sawing method by using diamond sawing is known from the document US 2005/0003633 A1. It is simultaneously established in the document that diamond sawing is problematic, particularly at the exit of the saw blade on the rear side of the semiconductor wafer. The rear side has high internal strains, especially as the rear side has less planarity and greater roughness than the active top side of the semiconductor wafer, such that chipping, microcracks and/or crystal defects occur when the semiconductor wafer is separated into individual semiconductor chips.
Consequently, the separation of semiconductor wafers by using diamond sawing is problematic and not satisfactory. In particular thin semiconductor wafers are moreover mechanically damaged by the sawing forces, which generate a high stress state in the crystal, very rapidly on account of microcracks and strains due to chipping. This becomes apparent particularly when the entire thickness of the semiconductor wafer is to be severed.
This problem is partly circumvented by using a semiconductor wafer being only incipiently sawn by introducing separating joints along the separating tracks. The semiconductor wafer is subsequently thinned by grinding from the stress-loaded rear side to an extent such that it separates into individual semiconductor chips. This procedure is also referred to as “dicing before grinding” or the DBG technique. Methods of this type require additional manufacturing processes and significantly higher manufacturing costs. In addition, problems remain unsolved, such as a slow separating speed, a non-stable cut quality and a high consumption of diamond saw blades, which are fundamentally associated with the diamond sawing of semiconductor crystals.
In order also to enable the entire semiconductor wafer to be sawn through in fracture-free fashion by using diamond saw blades, in the methods disclosed in the document US 2005/0003633 A1, the stress-loaded rear side of the semiconductor wafer is very largely leveled at least in the region of the separating tracks by using laser removal or by using laser melting of the rear side material and is annealed with low stress by using the laser treatment. This laser pretreatment has the effect that diamond sawing along the separating joints leads to improved edges of the semiconductor chips in the respective semiconductor chip positions.
The document JP 19860178392 discloses a laser separating method for improving the quality during the formation of separating joints in a semiconductor wafer by firstly introducing sawing joints by using a saw blade, which are then extended as far as the rear side of the semiconductor wafer by using laser light. The semiconductor wafer is separated by melting in this case. For this purpose, the semiconductor wafer is applied to a self-adhesive surface of a self-adhesive film including a UV-curable resin adhesive. The semiconductor wafer is then fixed on the film by using thermocompression bonding. Finally, the separating joints are sawn in along the separating tracks of the semiconductor wafer and the further wafer material is exposed to a laser cutting device and separated by projection of the laser light.
Since the semiconductor material not separated by the saw is separated by laser light, fractures of the semiconductor chips during removal from the film are prevented. In addition, the laser separation makes it possible to shorten the processing time and simultaneously to improve the cutting quality for the semiconductor wafer. However, two technically different separating methods are combined, which puts a burden on the manufacturing costs.
The document JP 19870527 discloses a similar method, which involves firstly effecting sawing to a predetermined depth by using a diamond saw along the separating tracks, and then severing the remainder, which amounts to approximately 20 micrometers, by using a laser in order not to produce any chipping in the bottom region. For this purpose, the laser is guided in a water jet and at the same time the diameter of the laser beam is set to be smaller than the thickness of the separating saw blade.
This water jet guidance for the laser beam has the disadvantage that this manufacturing requires complicated measures for carrying away the volume of water that arises. In some instances it is necessary to use specially porous and water-permeable films, which impede further manufacturing processes, so that such porous and water-permeable films also have to be removed again from the separated semiconductor chips before further processing.
Silicon wafers that are singulated by a laser process nevertheless have a greatly reduced breaking strength. A reduced breaking strength can lead to a semiconductor chip fracture in the subsequent manufacturing and mounting processes, such as semiconductor chip bonding, bond wire bonding, injection-molding processing or soldering, such that unacceptable manufacturing rejects arise.
FIGS. 12 to 15 illustrate this problem of laser cutting in the prior art.
FIG. 12 illustrates a schematic cross section through a portion 13 of a semiconductor wafer 1 in the region of a separating track 6 arranged between two semiconductor chip positions 5 of the semiconductor wafer 1. The width b of the separating track 6 is marked in an optically visible manner by corresponding edge structures 10 and 11 on the top side 12 of the semiconductor wafer 1. Semiconductor component structures associated with the respective semiconductor chip positions 5 are arranged in the regions 14 near the surface of the semiconductor wafer 1.
FIG. 13 illustrates a schematic cross section through the portion 13 in accordance with FIG. 12 during irradiation of the separating track 6 by a laser ablation beam 9, which brings about material removal in the cross hatched region with crystallographic strain 7 in the semiconductor wafer 1 along the separating track 6.
FIG. 14 illustrates a schematic cross section through the portion 13 in accordance with FIG. 12 after removal of part of the semiconductor material to form a separating joint 15 in the region of the separating track 6. This gives rise to an amorphous region 21 made of semiconductor material at the edges 16 and 17 of the separating joint 15, which region arises as a result of momentary melting of the edge sides 16 and 17 during laser irradiation. In terms of the mass density and in terms of further mechanical properties, such amorphous silicon differs significantly from the monocrystalline silicon 22 of which the semiconductor wafer 1 is composed. This produces, at the boundary or in the region of the edge sides 16, 17, excessive amorphous-crystalline-mechanical stress increases that can lead to the nucleation of cracks and to the reduction of the flexural breaking strength of the semiconductor chip that arises.
FIG. 15 illustrates a schematic cross section through the portion 13 in accordance with FIG. 14 after the severing of the semiconductor wafer 1 with the jeopardized, amorphously solidified edges 16 and 17 of the separating joint, the profile 18 of the crystallographic strain being illustrated as a diagram. It becomes clear that the highest strain 23 lies in the transition from the amorphous region 21 to the monocrystalline region 22. Such amorphization cannot be prevented even by a water jet and thus by water jet conducted and cooled laser beam removal, especially as the amorphous state of an inherently crystalline semiconductor material is effected only by quenching of a melt and hence solidification in the melt-like state.